1. Technical Field
The present invention relates generally to a fan drive clutch for an internal combustion engine. More specifically, the invention relates to a temperature sensitive type `an drive clutch including a fluid coupling for controlling the rotational speed of a fan.
2. Background Art
Use of temperature sensitive type fan drive clutches in automotive cooling systems is well known in the art. This type of fan drive clutch generally includes a fluid coupling which couples a drive shaft with a clutch housing on which a fan is disposed. The fan is operated according to the air temperature required by an automotive radiator. The fluid coupling allows the fan to rotate, blowing air into the radiator. The fan drive clutch is adapted for controlling the rotational speed of the fan so as to increase the fan speed when engine cooling requirements are severe and to drop the fan speed when the engine cooling requirements are low.
Internally, the fluid coupling is provided with a pump plate for defining a fluid reservoir chamber and a working chamber, a clutch plate, a valve arm, and a bimetallic coil for controlling the movement of the valve arm. The clutch plate has labyrinth grooves in its outer end section for engaging with corresponding labyrinth grooves formed in the inner wall of the housing. The pump plate has a through hole for allowing working fluid to pass therethrough from the reservoir chamber to the working chamber. The valve arm includes a valve plate for opening and closing the through hole to control the flow rate of the working fluid from the reservoir chamber into the working chamber. The valve arm is rotatably attached to the clutch housing by means of a shaft which is inserted into a bearing hole provided in the clutch housing. The bimetallic coil is connected to the valve arm and is operable to move the valve arm so as to slide it through the bearing hole according to variations in the temperature of the air passing through the radiator. This action causes a through hole to be opened and closed, the working fluid thereby flowing into the working chamber. When more working fluid is supplied into the labyrinths to couple the drive shaft with the clutch housing, more torque from the drive shaft is transmitted to the clutch housing, and therefore fan speed is increased.
Conventional fan drive clutches usually require location of the valve plate as close to the pump plate as possible in order to close and open the through hole hermetically. However, in a conventional clutch, a certain clearance is provided between the shaft for supporting the valve arm and the bearing hole formed in the clutch housing on account of the assembly tolerance required. Thus, when the shaft moves within the clearance, the valve arm tends to swing according to the motion of the shaft. For example, the top edge of the valve plate can be forced against a surface of the pump plate, while the opposite edge thereof is lifted from the surface of the pump plate causing a relatively large gap to occur. As a result, the working fluid in the reservoir chamber flows into the working chamber through the gap and the through hole. The working pressure within the working chamber is therefore increased. As a result, even when engine cooling requirements are low, some torque from the drive shaft tends to be transmitted to the housing, causing the fan to rotate. Thus, the radiator can be overcooled during engine warm-up.